Inhalation/exhalation respiratory phase detection circuit

ABSTRACT

An apparatus (10) for controlling the pressure of a respiratory gas delivered to a patient includes a phase detection circuit (24) for determining the inhalation and exhalation phases of the patient&#39;s respiratory cycle. More particularly, a flow signal representative of the respiratory flow is compared to second signal, offset in time and scaled in magnitude relative to the flow signal, in order to determine the transition from one phase to the next.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is concerned with an apparatus for controlling thepressure of a respiratory gas delivered to a patient. More particularly,the preferred apparatus includes a trigger circuit for determining theinhalation and exhalation phases of the patient's respiratory cycle.

2. Description of the Prior Art

Obstructive sleep apnea is a sleep disorder characterized by relaxationof the airway including the genioglossus throat muscle during sleep.When this occurs, the relaxed muscle can partially or completely blockthe patient's airway. Partial blockage can result in snoring orhypopnea. Complete blockage results in obstructive sleep apnea.

When complete blockage occurs, the patient's inhalation efforts do notresult in the intake of air and the patient becomes oxygen deprived. Inreaction the patient begins to awaken. Upon reaching a nearly awakenedstate, the genioglossus muscle resumes normal tension which clears theairway and allows inhalation to occur. The patient then falls back intoa deeper sleep whereupon the genioglossus muscle again relaxes and theapneic cycle repeats. In consequence, the patient does not achieve afully relaxed deep sleep session because of the repetitive arousal to anearly awakened state. People with obstructive sleep apnea arecontinually tired even after an apparently normal night's sleep.

In order to treat obstructive sleep apnea, a system of continuouspositive airway pressure (CPAP) has been devised in which a prescribedlevel of positive airway pressure is continuously imposed on thepatient's airway. The presence of such positive pressure provides apressure splint to the airway in order to offset the negativeinspiratory pressure that can draw the relaxed airway tissues into anocclusive state. The most desired device for achieving a positive airwayconnection is the use of a nasal pillow such as that disclosed in U.S.Pat. No. 4,782,832, hereby incorporated by reference. The nasal pillowseals with the patient's nares and imposes the positive airway pressureby way of the nasal passages. The nasal pillow also includes a smallvent for continuously exhausting a small amount of air in order toprevent carbon dioxide and moisture accumulation.

In the CPAP system, the patient must exhale against the prescribedpositive pressure. This can result in patient discomfort, especially atthe higher pressure levels. Because of this problem, the so-calledbi-level positive airway pressure (BiPAP) system has been developed inwhich the pressure is lowered during the exhalation phase of therespiratory cycle. Practical implementation of the BiPAP system has metwith only marginal success because of the difficulty in accurately andreliably detecting the occurrence of the exhalation and inhalationphases of the respiratory cycle. Respiratory phase detection has been aproblem because the continual air exhaust at the nasal pillow, and othersystem leaks, results in a net positive air flow to the patient. Thus,phase transition cannot be determined merely on the basis of a change inthe direction of air flow.

Summary of the Invention

The apparatus of the present invention solves the prior art problemsdiscussed above and provides a distinct advance in the state of the art.More particularly, the apparatus hereof reliably determines inhalationand exhalation phases in the respiratory cycle in order to controlrespiratory gas pressure in response.

The preferred embodiment of the invention hereof includes a gas supplyfor supplying a respiratory gas under pressure from a source thereof toa patient, a phase detection circuit for detecting the inhalation andexhalation respiratory phases, and a pressure controller for controllingthe pressure delivered to the patient in a predetermined mannercorrelated with the respiratory phases.

The preferred phase detection circuit produces first and second signalsrepresentative of respiratory gas flow with these signals being timedisplaced relative to one another and scaled in magnitude. With thisconfiguration, the signals present different gains and voltage offsetsrelative to one another during respective phases of the respiratorycycle. These two signals are compared to determine transitions, whichcorrelate with transitions from one respiratory phase to another. Withreliable phase detection, the gas pressure delivered to the patient iscontrolled in accordance with the phases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the preferred apparatus forfacilitating the respiration of a patient;

FIG. 2 is an electrical schematic of the preferred phase detectioncircuit of the apparatus of FIG. 1;

FIG. 3 is a graph illustrating the flow and offset signals of thedetection circuit of FIG. 2 and illustrating patient inhalation andexhalation phases;

FIG. 4 is an electrical block diagram illustrating the preferredpressure controller of FIG. 1;

FIG. 5 is an exploded perspective view of the major components of thepreferred valve of FIG. 1;

FIG. 6 is a lower perspective view of the inlet/outlet housing of thevalve of FIG. 5;

FIG. 7 is a partial sectional view of the assembled valve of FIG. 5illustrating the shiftable components in a first position;

FIG. 8 is a partial sectional view of the assembled valve of FIG. 5illustrating the shiftable components in a second position;

FIG. 9 is a perspective view of a second embodiment of the valve of FIG.1;

FIG. 10 is a cut-away perspective view of the valve of FIG. 9; and

FIG. 11 is a sectional view of the valve of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring initially to FIG. 1, apparatus 10 includes gas source 12,control valve 14, pressure sensor 16 and flow sensor 18 coupled with aso-called ADAM circuit available from Puritan Bennett Corp. of Lenexa,Kans., which includes pneumatic hose 20 and nasal pillow 22. Apparatus10 further includes phase detection circuit 24 and pressure controller26. In the preferred embodiment, components 12-18 and 24-28 are enclosedin a single housing to which the ADAM circuit is coupled.

Gas source is preferably a variable speed blower operable to produce 120liters per minute at 30 cm. water pressure. The preferred pressuresensor 16 is available from Sensym Company as model number SCX01. Flowsensor 18 is preferably model AWM2300 available from Microswitch Corp.,transducer operable for producing an electrical signal on line 30representative of the air flow therethrough and thereby representativeof the air flow delivered to the patient.

FIG. 2 is an electrical schematic of phase detection circuit 24 whichincludes signal production circuit 32 and signal processing circuit 34.Signal production circuit 32 receives the flow signal from flow sensor18 by way of line 30. This signal is filtered for noise and othertransients by resistor R1 (22K) and capacitor C1 (1 uF) connected asshown in FIG. 2 and delivered as signal "S" to signal processing circuit34.

Signal production circuit 32 also transforms the flow sensor signal intoan offset signal "Sd" which is delayed in time and scaled in magnituderelative to signal S. Initially the flow sensor signal is time delayedby 200 milliseconds using resistor R2 (100K) and capacitor C2 (2.2 uF)interconnected as shown. The relative time delay between time signals Sand Sd is illustrated by the graphs in FIG. 3.

The time delayed signal is then delivered to the positive input terminalof amplifier A1 (type 358A) with the output therefrom connected asfeedback to a negative input terminal. The output of amplifier A1 isalso connected to output resistor R3 (221K). Amplifier A1 functions as avoltage follower to provide a high impedance input to the flow signal.

The conditioned time delay signal is then processed to scale themagnitude thereof so that signal Sd presents lower amplitude than signalS during the inhalation phase, and so that signal Sd presents a higheramplitude than signal S during the exhalation phase as illustrated inFIG. 3. To accomplish this, the gain of the signal is changedindependently for the inhalation and exhalation portions of the signaland a variable offset is added by the sensitivity potentiometers R4 andR9.

As discussed further hereinbelow, the output from phase detectioncircuit 24 produces a logic high output during exhalation and a logiclow during inhalation. These outputs are also provided as feedback tosignal production circuit 32, specifically to control terminal C of CMOSinhalation switch S1 and to control terminal C of CMOS switch S2. TheseCMOS switches are type 4066B and operate so that when a logic high inputis provided to terminal C, the switch is "on," that is, connection ismade between terminals "I and "O" thereof. When terminal C is low, theconnection between terminals I and O is open.

During inhalation, terminal C of switch S1 is low and the switch is off.Voltage is then supplied to the negative input terminal of amplifier A2(type 358A) by way of inhale sensitivity potentiometer R4 (500 Ohms fullscale), resistor R5 (10K) and resistor R6 (221K). Resistor R7 (221K)interconnects the output of amplifier A2 with the negative inputterminal thereof. The level of the voltage delivered to negative inputterminal of amplifier A2 determines the amplitude scaling of the delayedflow sensor signal delivered to the positive input terminal. Morespecifically, potentiometer R4 is adjusted to provide the desired offsetof output signal Sd relative to signal S during inhalation.

Also during inhalation, the logic low signal is delivered to terminal Cof switch S2, which turns this switch off. In turn, a logic high signalis imposed on terminal C of CMOS switch S3 by way of resistor R8 (10K).This turns on switch S3 which imposes ground potential on the voltageoutput from potentiometer R9 and resistor R10 and thereby disables theexhale sensitivity portion of the circuit.

During exhalation, a logic high signal is delivered to terminal C ofswitch S1 which then turns on and imposes ground potential on thevoltage output from potentiometer R4 and resistor R5 in order to disablethe inhalation sensitivity portion of the circuit. The logic highexhalation signal also turns on switch S2 which imposes ground potentialon the voltage output from resistor R8. In turn, switch S3 turns off.This allows exhale sensitivity voltage to be delivered to the positiveinput terminal of amplifier A2 by way of exhale sensitivitypotentiometer R9 (500 Ohms full scale), resistor R10 (10K) and resistorR11 (221K). Potentiometer R9 is adjusted to provide the desired offsetof signal Sd relative to signal S during inhalation.

As illustrated in the graph of FIG. 3, signal production circuit 32produces signals S and Sd so that the voltage level of signal Sd is lessthan that of signal S during inhalation. Conversely, the voltage levelof signal S is less than that of signal Sd during exhalation.

Signal processing circuit 34 receives signals S and Sd and comparesthese signals to determine the occurrence of the inhalation andexhalation phases of the respiratory cycle. Specifically, signal S isreceived at the negative input terminal of comparator A3 (type 358A) andsignal Sd is received at the positive input terminal thereof by way ofresistor R12 (100K). When the voltage level of signal S is greater thanthat of signal Sd, the output from comparator A3 is logic low andinhalation is indicated thereby. When the voltage level of signal Sd isthe greater of the two, comparator A3 output goes high and exhalation isindicated.

Resistor R13 (100K), resistor R14 (10 m), and capacitor C3 (2.2 uF) areinterconnected with comparator A3 as illustrated in FIG. 2 and provide asignal blanking interval after a transition in the output of comparatorA3. More particularly, resistor R13 and capacitor C3 provide increasedvoltage hysteresis in the delivery of feedback from the output to thepositive input terminal of comparator A3 in order to eliminate falsetriggering due to transients, noise or the like. Capacitor C4 (100 nF)provides input smoothing for the supply voltage delivered to comparatorA3.

An inspection of the graphs of signals S and Sd in FIG. 3 illustratescrossover points 36 and 38, and artifact 40 at the inhalation peak insignal Sd. Crossover points 36,38 are determined by the time delayimposed by resistor R2 and capacitor C2, by the amplitude scaling, andby the offset voltages which can be adjusted by potentiometers R4 and R9for the respective phases. Artifact 40 corresponds to the phase changefrom inhalation and exhalation, and occurs because of the transition ofsignal production circuit 32 between the inhalation and exhalationoffset modes. The time delay from crossover 36 to artifact 40corresponds to the blanking interval determined by the hysteresis ofcomparator A3 as set by resistors R12, R13 and R14. Phase detectioncircuit 24 provides an output on line 42 representative of theinhalation and exhalation phases of the patient. More particularly,circuit 24 provides a logic high output at +10 VDC during exhalation anda logic low output at 0 volts during inhalation.

FIG. 4 is an electrical block diagram illustrating pressure controller26, control valve 14 and pressure sensor 16. In general, controller 26receives signals from phase detection circuit 24 and pressure sensor 16and, in response, operates valve 14 to maintain the respectiveinhalation and exhalation pressures delivered to the patient.

Pressure sensor 16 provides a pair of differential voltage signals tothe corresponding inputs of differential amplifier 44 that responds byproviding a voltage output (Vp) to error detector 46 representative ofthe pressure being delivered to the patient. Conventional error detector46 compares the pressure signal Vp with a set point pressure signal Vsin order to produce error signal Ve.

Set point signal Vs is produced by digital-to-analog converter (DAC) 48,DAC 50 and CMOS switch 52. DAC 48 receives a digital inputrepresentative of the desired exhalation positive air pressure (EPAP) byway of a set of five DIP switches 54, and converts the digital output toa representative analog signal delivered to terminal I2 of switch 52.Similarly, DAC 50 receives its digital input for inhalation positive airpressure (IPAP) from a set of five DIP switches 56, and delivers itsanalog output to terminal I1 of switch 52. Control terminal C isconnected to line 42 and receives the inhalation and exhalation signalsfrom phase detection circuit 24. During exhalation, the +10 VDC signalreceived at terminal C activates switch 52 to provide the EPAP voltageat terminal I2 as the output Vs. During inhalation, the logic low signalat terminal C causes switch 52 to provide the IPAP voltage at terminalI1 as the output Vs.

Error signal Ve is provided to interface 58 which is a conventionalinterface circuit designed to transform error signal Ve into a signal Vccompatible with valve 14 according to the specifications supplied by themanufacturer. Signal Vc is delivered to power amplifier 66 and isinverted as a corresponding input to power amplifier 68. The net resultis a differential voltage output from amplifiers 66 and 68 which isdelivered to the terminals of the valve motor of control valve 14, asexplained further hereinbelow.

FIGS. 5-8 illustrate preferred control valve 14, which includes valvebase 70, shiftable valve element 72 and valve element cover 74. Valvebase 70 includes housing 76 and valve motor 78 having motor shaft 80with locking hole 81 defined in the end thereof.

Housing 76 is preferably composed of synthetic resin material having agenerally cylindrical configuration and presents upper and lowersections 82 and 84. Upper section 82 includes upper face 86 havingcentrally defined opening 88 for receiving motor shaft 80, Which extendsupwardly therethrough. Sidewalls 90 of upper section 82 present aslightly smaller diameter than sidewalls 92 of lower section 84 andthereby define shelf 94 for supporting valve cover 74. Housing 76 alsoincludes three, outwardly and upwardly opening recesses 96a, 96b and 96cpresenting a generally trapezoidal configuration in cross section. Eachrecess is defined by lower wall 98, and side walls 100, 102 and 104.Additionally, upper section sidewalls 90 include three outwardly lockingbosses 106 located midway between adjacent recesses 96a-c.

Integral valve element 72 includes frusto-conically shaped hub 108,support ring 110, three, pie-shaped, equally spaced, support bodies114a, 114b and 114c interconnecting hub 108 and support ring 110, andthree, rectangularly shaped valve fingers 116a, 116b and 116c equallyspaced about the periphery of hub 108 and extending upwardly therefrom.Hub 108 includes hole 118 defined in the lower surface thereof forreceiving motor shaft 80. Additionally, hub 108 includes aperture 120centrally defined through the upper surface thereof for receiving alocking screw therethrough which is further received in motor shaftlocking hole 81 for securing element 72 to shaft 80. Hub 108, ring 110and support bodies 114a-c define three, equally spaced, exhaust ports122a, 122b and 122c presenting a shape congruent with recesses 96a-c andconfigured for registration therewith.

Valve element cover 74 includes inverted cup shaped member 124,presenting sidewall 126 and top wall 128, and further includes inlettube 130, outlet tube 132 and valve fingers 134a, 134b and 134c. Inlettube 130 is coaxial with cup shaped member 124 at top wall 128 whileoutlet tube 132 extends outwardly from sidewall 126. Equally spacedfingers 134a-c depend downwardly from inner surface 136 of top wall 128and are configured intercalate with fingers 116a-c and with the spacestherebetween. Tubular member further includes spaced slots 138 definedin the lower edge of sidewall 126 and configured to register with acorresponding locking boss 106 in order to secure cover 74 to valve base70.

FIGS. 7 and 8 illustrate assembled control valve 14 with valve fingers134a-c of cover 74 fitting concentrically about valve fingers 116a-c ofrotatable element 72. In operation, pressure controller 26 energizesvalve motor 78 in order to rotate element 72 clock-wise or counterclock-wise between a fully closed position (FIG. 7), a fully openedposition (FIG. 8), and intermediate positions therebetween.

In the fully closed position of FIG. 7, fingers 116a-c and 134a-c arefully meshed in order to block the respective spaces and ports 122 arein complete registration with recesses 96a-c. In this position, all ofthe air entering inlet tube 130 from source 12 exhausts through ports122a-c and recesses 96a-c. In the fully open position of FIG. 8, fingers116a-c and 134a-c are in registration so that the spaces therebetweenare open, and support bodies 114a-c are in registration with and therebyblock recesses 96a-c. With this orientation, all of the air is exhaustedthrough outlet tube 132 for delivery to the patient.

The intermediate positions between fully opened and fully closed allowrespective portions of the inlet air to exhaust through recesses 96a-cand through outlet tube 132. In this way, control valve 14 provides moreprecise control over the pressure delivered to the patient, and providessmoother transition between pressure settings.

In the operation of apparatus 10 during inhalation, it is necessary toprovide sufficient pressure to maintain the airway pressure splint inthe patient in order to prevent occlusion. For patient comfort, however,it is desirable to lower the pressure to a level as low as possible,including ambient pressure, while still maintaining sufficient pressureto keep the airway open. In order to accomplish these benefits, phasedetection circuit 24 detects the inhalation and exhalation phases of thepatient's respiration, and provides corresponding outputs to pressurecontroller 14. In the preferred embodiment, controller 14 controls itsoutput pressure in a predetermined manner correlated with inhalation andexhalation as indicated by the outputs received from circuit 24. Moreparticularly, pressure controller 14 controls the pressure delivered tothe patient at a higher level during inhalation and a lower level duringexhalation as determined by the settings on DACs 48,50. Typically, therespective inhalation and exhalation pressure levels are prescribed bythe patient's physician.

FIGS. 9-11 illustrate control valve 140 which is another embodiment of acontrol valve for use in place of valve 14. Valve 140 includes valvebody 142 and actuator assembly 144. Valve body 142 includes externaltubular inlet coupler 146 in communication with inlet passage 148, andfurther includes exhaust passage 150 and outlet passage 152 havingexternal outlet coupler 154 extending therefrom. As illustrated in FIGS.10-11, exhaust and outlet passages 150,152 communicate with inletpassage 148 and extend transversely therefrom, parallel to one another.

Actuator assembly 144 includes valve motor 156, valve stem 158, exhaustvalve element 160 and outlet valve element 162. As illustrated in FIGS.9-11, motor 156 is coupled to the bottom of body 142 with motor-actuatedstem 158 extending upwardly therefrom through, and transverse to,exhaust and outlet passages 150,152. Valve elements 160,162 presentoval-shaped configurations and are coupled with stem 158 for rotationtherewith. Element 160 is positioned in exhaust passage 150, and element152 is positioned in outlet passage 152. Valve elements 160,162 functionin a manner analogous to conventional butterfly valves. As illustrated,valve elements 160,162 are angularly displaced from one another on stem158 by about 45°. Valve motor 156 is coupled electrically with pressurecontroller 26 and receives signals therefrom in the same manner as valvemotor 78 or valve 14.

FIGS. 10 and 11 illustrate control valve 140 in the closed/exhaustposition. In this position, exhaust valve element 160 is positionedparallel to the air flow and outlet valve element 162 is positioned sothat its edges engage the sidewalls of outlet passage 152 to block alloutflow. In other words, all of the inlet air entering through inletpassage 148 would exhaust through exhaust passage 150 and none would beprovided through outlet passage 152 to the patient. In the open/outletposition, elements 160 and 162 would be rotated clockwise as viewed fromabove until the edges of exhaust element 160 engage the walls definingexhaust passage 150. In this position, outlet element 162 is positionedparallel to the air flow through outlet passage 152. In this way, no airis exhausted but rather, the full supply is provided through outletpassage 152.

Motor 156 responds to the signals received from pressure controller 26in order to position valve 140 in the closed or open positions or anyintermediate position therebetween. As with control valve 14, thisarrangement allows smooth controllable transition between the variousvalve positions.

Having thus described the preferred embodiment of the present inventionthe following is claimed as new and desired to be secured by LettersPatent:
 1. An apparatus for detecting the inhalation and exhalationphases of a respiratory cycle having associated respiratory gas flow,said apparatus comprising:signal production means for producing firstand second signals representative of the respiratory gas flow with saidsignal production means further including means for delaying one of saidsignals in time relative to the other of said signals; and said signalshaving respective amplitudes so that one of said signals presents thegreater amplitude during at least a portion of one of the phases and sothat the other of said signals presents the greater amplitude during atleast a portion of the other of said phases; and processing means forprocessing said signals for determining therefrom the occurrence of saidrespective phases and for producing outputs representative of saidphases.
 2. The apparatus as set forth in claim 1, said signal productionmeans including flow sensor means for producing said first signalrepresentative of instantaneous respiratory gas flow and time delaymeans for producing said second signal delayed in time relative to saidfirst signal.
 3. The apparatus as set forth in claim 2, furtherincluding amplitude scaling means for producing said second signalscaled in amplitude relative to said first signal.
 4. The apparatus asset forth in claim 3, said scaling means including means for scalingsaid amplitude at a first level during one of said phases and means forscaling said amplitude at a second level during the other of saidphases.
 5. The apparatus as set forth in claim 4, wherein said means forscaling said amplitudes at a first level and a second level areaccessible by a patient.
 6. The apparatus as set forth in claim 1, saidprocessing means including means for comparing the amplitudes of saidfirst and second signals, for producing a first output when said firstsignal presents a greater amplitude than said second signal, and forproducing a second output when said second signal presents a greateramplitude than said first signal.
 7. The apparatus as set forth in claim1, said first and second signals being electrical signals.
 8. Theapparatus as set forth in claim 1, said outputs including electricalsignals.
 9. An apparatus for facilitating the respiration of a patienthaving respiratory cycle with associated respiratory gas flow andexhibiting inhalation and exhalation phases, said apparatuscomprising:supply means for supplying a respiratory gas under pressurefrom a source thereof to a patient; means for producing first and secondsignals representative of the respiratory gas flow with said signalproduction means further including means for delaying one of saidsignals in time relative to the other of said signals; and said signalshaving respective amplitudes so that one of said signals presents thegreater amplitude during at least a portion of one of the phases and sothat the other of said signals presents the greater amplitude during atleast a portion of the other of said phases; processing means forprocessing said signals for determining therefrom the occurrence of saidrespective phases and for producing outputs representative of saidphases; and control means coupled with said supply means and saidprocessing means for receiving said outputs and responsive thereto forcontrolling said respiratory gas pressure to the patient in apredetermined manner correlated with said phases.
 10. The apparatus asset forth in claim 9, said control means including means for controllingsaid respiratory gas pressure at a first pressure level during theinhalation phase and for controlling said respiratory gas pressure at asecond pressure level lower than said first level during the exhalationphase.
 11. The apparatus as set forth in claim 10, said second pressurelevel including ambient pressure.
 12. The apparatus as set forth inclaim 10, said second pressure level including a pressure level greaterthan ambient.
 13. The apparatus as set forth in claim 9, said signalproduction means including flow sensor means for producing said firstsignal representative of instantaneous respiratory gas flow and timedelay means for producing said second signal delayed in time relative tosaid first signal.
 14. The apparatus as set forth in claim 13, furtherincluding amplitude scaling means for producing said second signalscaled in amplitude relative to said first signal.
 15. The apparatus asset forth in claim 14, said scaling means including means for scalingsaid amplitude at a first level during one of said phases and means forscaling said amplitude at a second level during the other of saidphases.
 16. The apparatus as set forth in claim 15, wherein said meansfor scaling said amplitudes at a first level and a second level areadjustable by a patient.
 17. The apparatus as set forth in claim 9, saidprocessing means including means for comparing the amplitudes of saidfirst and second signals, for producing a first output when said firstsignal presents a greater amplitude than said second signal, and forproducing a second output when said second signal presents a greateramplitude than said first signal.
 18. The apparatus as set forth inclaim 9, said first and second signals being electrical signals.
 19. Theapparatus as set forth in claim 9, said outputs including electricalsignals.
 20. A method for detecting the inhalation and exhalation phasesof a respiratory cycle having associated respiratory gas flow, saidmethod comprising:producing first and second signals representative ofthe respiratory gas flow from a signal production means, delaying one ofsaid signal in time relative to the other of said signals, said signalshaving respective amplitudes, said first signal presents the greateramplitude during at least a portion of the inhalation phase, said secondsignal presents the greater amplitude during at least a portion of theexhalation phase; and processing said signals for determining therefromthe occurrence of said inhalation and exhalation phases and forproducing a first output corresponding to the inhalation phase and asecond output corresponding to the exhalation phase.
 21. The method asset forth in claim 20, wherein said step of producing said first andsecond signals includes:using a flow sensor for producing said firstsignal, said first signal being representative of instantaneousrespiratory gas flow; and using time delay means for producing saidsecond signal delayed in time relative to said first signal.
 22. Themethod as set forth in claim 21, further comprising scaling said secondsignal in amplitude relative to said first signal.
 23. The method as setforth in claim 22, wherein said step of scaling said second signalincludes:using scaling means for scaling the amplitude of an inhalationphase of said second signal to a first level; and using said scalingmeans for scaling the amplitude of an exhalation phase of said secondsignal to a second level.
 24. The method as set forth in claim 23,wherein said step of scaling said second signal includes making saidscaling means accessible to a patient.
 25. The method as set forth inclaim 20, wherein said step of processing said signals furtherincludes:comparing the amplitude of said first signal and second signal;producing said first output when the amplitude of said first signal isgreater than the amplitude of said second signal; and producing saidsecond output when the amplitude of said second signal is greater thanthe amplitude of said first signal.
 26. The method as set forth in claim20, wherein said step of producing said first and second signalsincludes:producing said first signal as an electrical signal; andproducing said second signal as an electrical signal.
 27. The method asset forth in claim 20, wherein said step of processing said signalsincludes;producing said first output as an electrical signal; andproducing said second output as an electrical signal.